So I was sitting at my desk today, gulping cold coffee and working on an item for an end-of-year/ looking-ahead feature in our upcoming issue of Chief Marketer magazine about Google’s plans to become King of All Monetizable Media. I had just written that Google’s interest in mobile marketing extends beyond search to delivering display ads of all kinds, when the news came over the wire that the company will purchase mobile ad network AdMob for $750 million in stock, regulators willing.

Hey, I should be grateful. Companies usually wait to announce acquisitions like that until print magazines have been in the mail for 24 hours. Those of us who still have print deadlines can sometimes get paranoid about brands’ uneering ability to blow our news angles once too late to do anything about it.)

In this case, while the specific acquisition of AdMob was not predictable, the notion that Google intended to stake its claim to mobile marketing had been out there in the ether at least since May 2008, when CEO Eric Schmidt said in an interview that “The most obvious large space of advertising is the mobile Internet.”

“Just take the success of the iPhone,” he told a reporter at that time. “It has the first really powerful Web browser on a mobile device—and many more are still coming. By these products, the advertising gets more targeted because phones are personal. So targeted ads are possible. And that means the value of the ads will grow. The next big wave in advertising is the mobile Internet.”

Of course, that big wave will still constitute a relatively small ripple next to Web ad spending. Estimates from marketing research firm eMarketer suggest that mobile ad spending (of all types—display, search and SMS) by U.S. advertisers will reach $416 million this year and $1.6 billion by 2013, compared to $22.8 billion for Internet ads this year and $31.5 billion in four years.

But it’s about dominating a growth market. Under that forecast, mobile ads will increase 275% in the next four years, against about 38% for Web ad spending. And with those new smartphones—including those built on Google’s own Android platform– making up an increasingly dominant portion of the mobile installed base, mobile advertising might be able to sustain that growth rate for a good while to come.

Google has already been active in promoting mobile search to customers, and mobile search ads to marketers. In fact, earlier this week the company announced a deal that will have sales associates at Best Buy Mobile help customers download and optimize the free Google Mobile App on their new phones before they walk out the door—just to make sure that Google can capture that portion of the cellphone public that can’t navigate a download straight from the m.google.com site.

AdMob, started in 2006 and the beneficiary of several rounds of venture funding in that time, delivers ads to the cellphones that access the thousands of Web publishers who are part of its mobile ad network; mobile advertisers using AdMob right now include Ford and Coca-Cola.

AdMob is the largest network serving ads to iPhones, and that’s the sweet spot. The company has also begun delivering ads to phones built on the Android platform too, including the new Droid that was released to the Verizon network last week.

Even in the mobile ad ecosystem, display ads—the banners and other graphical ads that get served up along with Web pages– make up a smaller portion of the market than either search or text messaging: perhaps a quarter of total mobile ad spending today, according to eMarketer. But that proportion could grow to more than one third of mobile ad spending by 2013 as more smartphones users start accessing the Web more often from their handsets.

“AdMob has really big reach with iPhone users, and all the data suggest that these owners are a very attractive target, demographically and from the perspective of both income and phone usage,” says eMarketer senior analyst Noah Elkin. “Google clearly sees this combination of greater smartphone usage driving greater mobile Web usage and greater application usage.”

“And as on the desktop, it’s really going to be that twin bill of display and search that’s really going to drive mobile ad spending over the next several years,” he says.

And of course spending of this level by Google—AdMob is its third biggest buy in its ten-year history, after DoubleClick and YouTube—can help put a higher gloss on mobile marketing generally for advertisers and agencies strategizing campaigns. Kind words and rosy predictions of future use are nice, but nothing says confidence like a big check (or a whole lotta stock, as in this case.)

That’s the view AdMob founder and CEO Omar Hamoui took in an interesting blog post about the acquisition. Speaking of two mobile-based start-ups he was involved with before AdMob, he says the adage around the mobile water cooler years ago used to be “Mobile is the future—and always will be.”

The iPhone changed that from the user perspective, and Google’s growing involvement in ad formats will change it even further for marketers, according to Hamoui: “Until now, it has always felt like those of us involved in this [mobile] space played second fiddle to our online brethren. I believe that time is now over.”

What do you think? Will mobile continue to get more lip service than ad dollars? Or do its growing reach and consumer usage token the rise of a new channel ready to compete with other digital media, the Web and e-mail, for marketing budget?

Let me know. I’ll be at my desk. Although I think I’ll make a fresh pot of coffee.